When many gardeners hear the term "hardiness" they think of cold weather. Cold hardiness is a major factor that helps determine the survival of plants in our beds and borders in temperate regions. The United States Department of Agriculture (USDA) has developed a map that shows different zones based on their typical minimum winter temperatures.

Cold hardiness zones are most closely relevant to woody plants. The temperatures cited are air temperatures that are experienced by exposed woody trunks and stems. Through various "hardening" techniques, these plant tissues have become adapted to surviving in cold environments.

Plants such as herbaceous perennials and bulbs have adapted to cold winters by having their above ground tissues die down to the ground each autumn. They survive underground because of the insulating quality of the soil. A few inches beneath the soil surface, temperatures rarely go below the twenty to thirty degree range even if the air temperatures drop to subzero readings. If there is a covering of snow, the temperatures are even further moderated for the crowns, roots and bulbs in the ground.

More and more attention is being given to other types of plant hardiness. In some regions of the world, the key challenge for plants is to survive extremely hot, dry summers. Exposure to winter winds which will dry out exposed plant tissue may also be a challenge regardless of the minimum air temperature. Other areas experience basically two season per year, one wet and one dry.

Plant catalogs, labels and references in books or on the internet will provide you with the hardiness zone information for nearly all ornamental plants available today. Generally, these will be based on long-term observations of the plants in the landscape while a few may be based on scientific research at universities. Occasionally, newly introduced plants will be rated based on the climate at their native land which may or may not relate to your home's environment.

Remember that, regardless of its source, these ratings are generalizations. Your specific landscape site might have certain "microclimates" which may actually be milder or harsher that your USDA Zone would indicate.

For instance, if your property has a hill which slopes down into a very low lying area, this may have an impact on the hardiness of your plants. Cold air will settle down into the low area which may then experience temperatures several degrees cooler than at the top of the hill. This is especially important in the early spring when a frost may wipe out flower buds and new growth on plants down low and not bother those at the top of the hill.

Also, the direction and intensity of winds on your property may cause a discrepancy in hardiness levels. Some plants such as rhododendrons may be hardy for cold temperatures in your region but may be severely damaged by being exposed to high velocity winter winds. These will dry out the foliage and the frozen ground will not allow the plant to replenish them until it thaws.

Finally, a plant's hardiness level is often related to its general health and vigor as it enters the difficult weather period. Plants stressed by drought, poor nutrition or improper growing conditions may be less able to survive weather stresses. They may be generally hardy for your USDA Zone but, if they are not in good shape coming out of the summer, they may suffer winter damage anyway.

A note on the effects of global warming: Traditionally, the concept of cold hardiness is based on a one time exposure to low temperatures. Just because the average temperature for an area may be on the rise does not automatically mean that the cold hardiness rating will change.

The average temperature may go up a few degrees and that will affect other aspects of the growing and blooming season of a plant. However, if just one night in the winter drops below the cold hardiness rating for a plant, it may still die. If the plant thrives for 364 days but is subjected to temperatures that cause its cells to freeze and to burst their solid cell walls on the other night…it is still dead.